These Are the 3 Heat Pump Heat Sources
How do you heat your home? Here in Atlanta, most people use a furnace powered by natural gas. If you've got a furnace, you'll turn it on each winter, get toasty, and wait for the gas bill to arrive.
But there's another type of heating system that's almost as common. This system, which is powered by electricity instead of gas, is called a heat pump.
You usually know you've got one when your air conditioner's outdoor unit - the one that scares away the cats when you use your AC - also starts up when you set the thermostat to "heat." It's almost like your air conditioner is running in reverse. Come to think of it, heat pumps are like air conditioners that can run in reverse!
Here's another sign you've got a heat pump instead of a furnace: Your gas bill doesn't spike in winter or you don't have a gas bill at all.
Not all heat pumps are the same. There are three common sources of heat for heat pumps.
We've already described one type of heat pump: an air source model. These heat pumps rely on the same indoor and outdoor units that your air conditioner uses. They're by far the most common heat pump type in and around Atlanta.
The other types are ground source and water source heat pumps. If you've ever heard of geothermal heating, that's a ground source heat pump. In our area, the water source types are common in high-rise condominium buildings.
Let's take a closer look at the three types of heat pumps to get a better understanding of the mechanics involved - and why you might opt for one type over another.
1. Air source heat pumps
If you're an Atlanta heat pump owner, you've probably got an air source heat pump. The heat it pumps into your home is extracted from the air outside, even during the winter when it feels like there isn't much heat to extract!
However, when outdoor temperatures sink to a certain threshold - usually around 40 degrees - many heat pumps struggle to remove heat from the air. To pick up the slack, the system activates something called heat strips. These large heating coils (think of them as a big toaster) get really hot and help the heat pump satisfy your thermostat setting on those really cold days and nights.
Heat strips are very expensive to operate. Since most heat pump systems depend on heat strips during the coldest times of year, they generally heat your home less efficiently than gas furnaces .
So, why do some people use an air source heat pump instead of a furnace? There are four possible reasons:
- The home doesn't have gas lines. When you don't have natural gas running to your home, you almost certainly have a heat pump instead... and it's probably an air source model.
- They actually do have a gas furnace, but... The heat pump is their primary heating source throughout the colder months. The furnace is there to supplement the heat pump in the dead of winter when outdoor temperatures are very low. This is called a dual fuel heating system, and it protects heat pump owners from using those expensive heat strips.
- They don't crank the heat in winter. If you're the kind of person who keeps the thermostat in the low 60s throughout the winter and bundles up to save energy, you won't need to use your heat strips very often. A heat pump system might actually be more efficient in your case!
- Heat pumps don't dry out the air as much as gas furnaces. Some people prefer heat pumps because they don't produce scorching heat like you get from a gas furnace. If you have sensitive skin that dries out a lot in winter, you might prefer a heat pump to a furnace.
How does an air source heat pump work?
Typical air source heat pumps are split HVAC systems with two copper coils: an indoor coil and an outdoor coil. The indoor coil is packaged within a larger unit called the air handler or blower, often found in a home's attic or crawlspace. It also contains a fan that moves air through your ductwork. The outdoor coil is housed within a unit outside the home. It's the part you can see and hear when it starts up.
In heating mode, the basic process of moving (that is, pumping) heat unfolds as such:
- Liquid refrigerant inside the outdoor coil removes heat from the ambient environment.
- The heat evaporates into a cold gas, and the system applies pressure to turn it into a hot gas
- A large fan moves air through the indoor unit; this air absorbs the heat and distributes it throughout the home.
- The gas condenses back into liquid refrigerant and the cycle repeats itself.
Here, the outdoor unit functions as an evaporator, and the indoor unit works as the condenser. In cooling mode, the same process operates in reverse - it's why an air conditioner's outdoor unit is always referred to as the condenser and the indoor unit is always referred to as the evaporator. When a heat pump is capable of cooling and heating, the different coils can reverse these roles.
2. Ground source heat pumps, aka geothermal heat pumps
So, most residential heat pump systems remove heat from the outdoor air and pump it into your home to keep you warm. However, there's another type of heat pump that removes air from the ground instead. It's called a ground-source heat pump, although you might know it by its more common name: geothermal heating.
Why use a geothermal heat pump instead of the more readily available air source type? Because they're way more efficient and last longer.
According to the EPA, geothermal heat pumps are up to 65% more efficient than your typical HVAC system. Not only that, the indoor components will provide around 25 years of life; the outdoor components last about a half-century. That's a much longer lifespan than a typical air source heat pump, which typically gives you 10 to 15 years of service.
Since the temperature in the ground ranges from the 40s to the 70s in practically all climates at practically all times of year, there's always plenty of underground heat for the system to use. This is why geothermal heat pumps are so much more efficient than air source models.
Remember how air source heat pumps use heat strips to supplement the heat pump when it's really cold outside? A geothermal system doesn't need to do that. There's enough heat in the ground for the heat pump to operate efficiently no matter what.
"Ok," you may be wondering. "If these ground source systems are so efficient and last so long, why doesn't everybody have one?" The answer is simple: installation.
Installing a ground source heat pump is very difficult and very costly. You basically have to tear apart your yard.
Since these systems remove heat from the ground, they require a subterranean ground loop filled with water. This water absorbs heat from the ground and ultimately transfers that heat to refrigerant inside a separate coil. Most of the time, these ground loops are closed systems where water inside the loop stays inside the loop. In some cases, ground loops are open systems in which water from a nearby source, like an underground well, regularly replenishes the loop.
Regardless of whether it's an open or closed system, ground loop placement and design depend on a number of factors, including the size of your yard, soil type, local climate, and groundwater locations. You might have to dig six feet deep or sixty feet deep. Maybe deeper.
No matter how you swing it, there will be some serious digging involved.
How a ground source heat pump works
After you've installed a geothermal or ground source heat pump, here's how it heats your home:
- Heat is absorbed by the water inside the ground loop.
- The heat is transferred to a refrigerant inside a separate coil; the refrigerant changes phases into a gas.
- As pressure is applied, the gas heats up. Air passes over the coil that contains the hot gas, distributing heat throughout the home.
- The pressure drops as the gas cools and condenses back into a liquid refrigerant.
Then the process repeats itself. If you're thinking that this system works a lot like an air source heat pump, you're right! The main difference is that the heat comes from the ground instead of the air.
3. Water source heat pumps
Assuming you have access to enough outdoor water , you could install a heat pump that removes heat from your home during the summer and moves it (the heat) to that outdoor water. In winter, that scenario could work in reverse... as long as there's enough heat inside the water to extract.
This type of heat pump is called a water source heat pump. In summer, the system rejects the heat (yes, "rejects" is the official term) into the water source. In the Atlanta area, water source heat pumps rely on outdoor cooling towers in summer and a supplemental heat source (a boiler, typically) that heats the water in winter.
In urban settings, water source heat pumps are typically found in high-rise condominiums. They're a good option for extremely dense residential developments.
Much as geothermal systems are more efficient than air source heat pumps due to the relatively high temperature of the ground, water source heat pumps are more efficient than air source heat pumps due to water's relatively high specific heat and ensuing high heat-transfer coefficients. If you live in a condominium in a large building, it's a pretty smart setup for HVAC.
How a water source heat pump works
Water source heat pumps work very much like their air source and ground source brethren:
- In winter, a water loop absorbs heat from the water source. A heat exchanger transfers heat from the water to liquid refrigerant.
- The refrigerant evaporates into a gas. Pressure inside the system raises the temperature of the gas.
- Air moves over the coil containing the hot gas, transferring heat to the home.
- The gas cools off and condenses back into liquid refrigerant; the cycle repeats itself.
During the summer, the process works in reverse. Heat is removed from the home and then "rejected" into the water inside the cooling tower.
So you know all about the 3 heat pump heat sources. Now what?
By this point, you ought to feel a little more informed about your heat pump! And if your home doesn't have a heat pump, you hopefully know what to expect if you ever end up with one.
Heat pumps, all in all, are great systems! Today's air source heat pumps are also much more efficient than what was available a decade back. Compared to old systems, modern air source heat pumps do a much better job of removing heat from the air on those colder days.
If you live in metro Atlanta and your heat pump is misbehaving, drop us a line! We service all makes and models of air source, water source, and geothermal heat pump systems.